This invention relates to a beam steering optical switch and in particular to a control system for a free-space optical cross-connect switch with actuation, such as piezoelectric actuation, or any other micro-optical positioning or beam-steering device.
Communication signals are now commonly transmitted across networks using compact optical fiber bundles that support substantial transmission capacity. Given the ever-increasing demands for improved signal quality and bandwidth, it is anticipated that fiber optic communications will continue to supplant copper wire based technologies for many years to come.
One of the reasons that fiber optic communication networks have attracted much attention relates to their higher bandwidth over previous network technologies. Fiber optic communication networks are composed of a large number of fiber optic lines that can carry many optical signals (e.g., Dense Wavelength Division Multiplexing (DWDM)). At junction nodes, where the fiber optic lines interconnect, the optical signals carried on these fiber optic lines are exchanged. The connections can be made by a variety of cross-connect switches, where any given optical signal on an input line brought to the junction can be switched to any output line at that junction under operation of a controller. So-called “all-optical” switches (i.e., with no optical-to-electrical conversions), that can switch signals while they are still in pure optical form, are an efficient and effective way to enable these functions in optical networks. To be effective, switches need to switch at high speed to support network provisioning, protection switching, and other network functions. However, it will be appreciated that there is a continuous desire to increase the speed of operation and reduce signal losses at these switch interfaces.
A typical all-optical free-space cross-connect switch consists of a fabric of optical emitters that launch a collimated optical beam, and another fabric of optical receivers. The emitters can be selectively connected to the receivers by varying the direction of the emitted collimated beam so as to impinge on a selected receiver. Any combination of active and/or passive emitters and/or receivers can be combined to form 1×N, N×1, N×N or M×N switch fabrics.
Many all-optical free-space cross-connect switches have been reported that either redirect a collimated beam that is launched in a fixed direction, or control the direction of a collimated beam. Switches that redirect a fixed collimated beam typically rely on an arrangement of micro-mirrors that can be tilted, typically by applying an electrostatic force. Switches that directly control the beam direction have optical elements that rotate, translate, or tilt in response to an applied actuation signal. The motion of the optical elements move the position of an optical emitter, such as a fiber tip, relative to the optical axis of a collimating lens, in order to vary the angle of the beam. Both types of optical switches can advantageously employ Micro-Electro-Mechanical Systems (MEMS) technology, with actuation provided by mechanical, electromagnetic, piezoelectric, photoactive ceramic or polymer, thermal, chemically-active polymer, electrostrictive, shape-memory alloy or ceramic, hydraulic and/or magnetostrictive actuators and other types of actuators known in the art.
There are a number of factors that limit the speed at which such optical switches can operate. One limitation results from the requirement to accurately align the ends of the fibers as they are moved to a new position. Precise alignment accuracy is required to minimize signal losses. In this regard, it will be appreciated that even slight misalignments of the fiber ends will result in a significant loss of the power of the transmitted optical signal and, potentially, of the information encoded in the communication signal. Moreover, switch designers are continuously striving to accommodate more fibers in smaller switches.
Recently, optical emitters with a controlled beam pointing direction have been proposed that incorporate actuators, such as piezoelectric or electrostrictive actuators. Actuators advantageously provide a fast response, produce large forces, have a high characteristic frequency for fast switching. Additionally, they are low-cost and have low susceptibility to vibration.
Various control techniques have been employed to control the beam pointing, thereby controlling the rate of switching, in free-space optical switches. For example, U.S. Pat. No. 6,484,114 issued to Dickson describes a method for calibrating a free space coupled fiber optic transmission system. The method uses signal content in a measure of the coupled power, specifically at frequencies equal to a sum, difference, and first harmonics of the frequencies of a control dither signal. A signal so detected can be used to infer and correct errors in a kinematic model used to apply the dithers. The technique described by Dickson is similar to other techniques that use open-loop oscillatory mirror commands (i.e., dithers) to detect alignment errors and/or intentional optical power attenuation.
In addition, U.S. Pat. No. 6,097,858 issued to Laor describes a control system for an optical switch that has a number of movable mirror surfaces that permit adjustment of the optical paths. In this approach, control signal receiving elements are separated, i.e., physically located away from, the ends of target fiber optics. This configuration, which is especially useful for stationary fiber ends, can be used in conjunction with targeting optics that use an off-path or secondary sensor provided as part of a fiber optic control system, where the off-path sensor uses a light source other than a signal, such a an information carrying signal, in the fiber optic light path.